It turned out that I misinterpreted the error: The actual error was wrong line breaks in the expected warning message. The protocol from Singular is printed to stdout, and apparently it was just by accident (though reproducible) that I saw it in the test log.

Note that the warning message misspells the word "guaranteed" (namely "guarantied"). I fixed that misspelling as well, and I also introduced nicer (I think) line breaks for the warning.